1. Field of the Invention
This invention relates to a method for compressing character or pictorial image data, and particularly to a data compressing method which contemplates to compress a quantity of data by means of a method for establishing sampling points and a method for establishing blocks as well as storing information for specifying outlines of a character, pictorial image or the like (hereinafter referred to simply as "character") in the case where the outlines of character are approximated with sets of functional curves or straight lines to effect data compression.
2. Description of the Prior Art
First of all, establishment of sampling points will be described hereinbelow. In a method for compressing character data wherein an outline of character is split with sampling points, and each of the outline segments thus split is approximated by means of functional curves or straight lines, it is important to establish a small number of sampling points as less as possible at a high rate within a range where characteristics in a shape of the character outline are not damaged.
Heretofore, as the simplest method for establishing such sampling points, there has been a method for determining sampling points by increasing variable every unit quantity with respect to the character outline developed on X- and Y-coordinates.
However, the establishment of sampling points in such method depends on only variable axis so that a prescribed precision is not guaranteed for a relationship between a curve (or a straight line) to be approximated and an actual character outline, and as a result, it was difficult to establish the most suitable sampling points.
Furthermore, there has been proposed such a method wherein precision between a curve (or a straight line) to be approximated and an actual character outline is determined in respect of candidate sampling points, then, the candidate sampling points are moved in a manner of trial and error in response to the precision determined, and the candidate sampling points are utilized as established sampling points at the time when a prescribed precision was obtained. However, since this method requires a trial and error process for establishing one sampling point, the processing therefor was redundant.
Next, compression of a quantity of data will be described hereinbelow.
It is well known that binary data determined by subjecting a character, pictorial image or the like (hereinafter referred to simply as "character") to dot-decomposition are data which possesses very high redundancy.
In this respect, various data compressing methods have heretofore been proposed for decreasing such redundancy. One example of these methods is a so-called outline method wherein shape of a character is grasped by whose outline, and information for specifying the outline is stored thereby contemplating to compress a quantity of data.
As data compressing methods according to such outline method, a vector approximating method as illustrated in FIG. 8, or an m-order curve approximating method as illustrated in FIG. 9 has already been proposed.
The vector approximating method illustrated in FIG. 8 relates to the ones which have been disclosed in Japanese Patent Laid-open No. 149522/1979 (U.S. Pat. No. 4,254,468) and Japanese Patent Laid-open No. 79154/1980, respectively.
An abstract of such vector approximating method is such that an outline 1 of arbitrary character indicated by a dotted line is approximated with a set of two-dimensional vectors 2, and specified information (position of starting point, length and inclination, or components in horizontal and vertical directions) for the respective vectors is utilized as memory data, whereby whose data compression becomes possible.
Furthermore, the m-order curve approximating method illustrated in FIG. 9 is the one which has been already filed by the present applicant and Japanese Patent Application No. 116,160/1980 was assigned to the application.
The abstract of this m-order curve approximating method is such that compression of a quantity of data is intended by storing coordinates of a group of points P established suitably on the outline of an optional character, and at the same time, it is contemplated to approximate a desired outline by means of a set of m-order curve elements 3 connecting (m+1) of arbitrarily continuous points with one another.
And these data compression methods according to such outline method have such characteristics that when interpolation processing or scale factor conversion processing of vector is carried out in the case where whose compression data are interpreted to reproduce a character image, the methods can cope with reproduction of character images of various scale factors.
On the other hand, however, these conventional data compressing methods in accordance with the outline method involve such an essential disadvantage in that the optimum result is not guaranteed in respect of smoothness of the outline (continuity in inclination of the outline).
More specifically, for instance, either inclination .delta. of respective left and right segments involving either each terminal outline point P of the vectors in FIG. 8, or each connecting point P.sub.c of the m-order curves 3 in FIG. 9 as its center is inevitably discontinuous.
In this respect, an outline shape of character has generally such a characteristic that not only the outline itself is continuous, but also the primary derived function (inclination of outline) as well as the secondary derived function (rate of change in inclination thereof vary continuously, if peculiar points such as intersecting portions of lines constituting a character, or extreme ends of so-called beaks corresponding to "hane" in Chinese or katakana character are excluded.
For this reason, conventional data compressing methods according to the outline method still contain such a problem that compression data being faithful to character outline cannot be obtained by the methods, besides unnaturalness (discontinuity in inclination) of a character image reproduced on the basis of such data cannot completely be removed.